Gas sensors are commonly employed in industrial and consumer applications to measure analytes in the gaseous state. Many gas sensors rely on the absorption characteristics of the target analyte when illuminated with radiation and comprise a radiation source, a detector capable of detecting radiation emitted by the radiation source, and a chamber for receiving the target gaseous analyte. Analyte gas within the chamber absorbs radiation of specific wavelengths or ranges of wavelengths and the attenuation of the radiation detected by the detector gives an indication of the concentration of the target analyte within the chamber. The target analyte typically diffuses into the chamber although sensors which actively transport gas into the chamber are known. In some optical absorption gas sensors, the source emits radiation at a broad range of wavelengths and a wavelength selective filter is provided at the detector, in which case the gas sensor is referred to as a non-dispersive sensor. In other optical absorption gas sensors, the source emits radiation of a defined wavelength or wavelength range or includes a filter which selects a specific wavelength band.
To achieve high sensitivity, it is desirable that the radiation has as long a mean path length as possible through the analyte, from the radiation source to the detector, such that the analyte absorbs a significant portion of the radiation, and that the radiation is not absorbed by any other processes within the sensor. For example, if the path between the radiation source and the detector is not linear, the radiation must be reflected to be directed to the detector. However, the reflecting surface will introduce optical loss from absorption and/or scatter and this optical loss will typically be significant in a low cost sensor. There is therefore a balance to be struck between increasing the number of reflections to increase path length and avoiding excessive attenuation of the emitted radiation by excessive reflections.
However, for many practical applications, it is desirable for gas sensors to be small, or to comply with standardised dimensions, resulting in generally short path lengths and corresponding low sensitivity. Thus, one aim of the present invention is to provide a gas sensor that has high sensitivity whilst being compact and suitable for cost-effective manufacture.
It is known to provide gas sensors having a straight hollow tubular radiation guide which extends between the radiation source and the detector and has a rectangular or other cross section. Radiation guides of this type guide radiation with a range of path lengths depending on the orientation at which radiation enters the radiation guide. Radiation which enters the radiation guide at a high angle of incidence to the walls of the radiation guide reflects many times and so is more highly attenuated than radiation which enters a straight rectangular radiation guide at an orientation close to the axis of the radiation guide.
It is known that for linear radiation guides increasing the size of the cross section decreases the number of reflections and therefore the absorption loss but results in poor collection efficiency. A better arrangement is to have the beginning and end of the waveguide shaped eg as compound parabolic collectors (CPCs). The effect of this is to transform the radiation field as it moves along the waveguide. At the source it has a large angular spread and relatively small spatial spread, midway it has a small angular spread but large spatial spread and at the detector again it has a large angular spread but small spatial spread. If chosen correctly this arrangement can significantly reduce the absorption due to multiple reflections.
The incorporation of a curved portion into a radiation guide can facilitate the provision of a longer path for radiation in a given volume than a radiation guide lacking curved portions. However, where a radiation guide curves, radiation falling on the curved walls of the guide results in greater angular spread without a reduction in spatial spread and consequently an increase in loss due to absorption in the multiple reflections. In particular, where radiation falls onto a curved wall, parallel radiation incident on the curved wall will not be parallel after reflection and so be dispersed. Some radiation will be incident on a curved wall with a greater angle of incidence than would be the case were the wall planar. Thus, where a radiation guide curves, some radiation is reflected into a path where it reflects at a relatively high angle towards an opposite wall, with the effect that a substantial proportion of radiation entering a curved radiation guide may be reflected many times and so be strongly absorbed.
Furthermore, in embodiments comprising a curved radiation guide and a collector at the detector, the collimator is unable to direct the radiation onto the detector due to the increase in angular spread. Therefore, the intensity of radiation incident upon the detector is reduced, correspondingly reducing the sensitivity of the gas sensor.
Some aspects of the invention aim to provide optical absorption gas sensors with improved curved radiation guides which better transmit radiation from a radiation source to a detector through a gas sample.
Another problem which arises in the field of absorption gas sensors is that during operation the radiation source, which is often an infra-red radiation emitter, and the detector are temperature sensitive. The emission spectrum of the radiation source may vary with temperature and the sensitivity of the detector may also vary with temperature. As well as being affected by ambient temperature, the radiation source in particular will heat up in use. If the radiation output is to be substantial and pulsed, temperature may fluctuate dramatically. When measuring small attenuations, small measurement errors due to temperature fluctuations in either or both the source and the detector can create substantial errors in measured gas concentration.
WO 2007/091043 (Gas Sensing Solutions Limited) discloses a sensor in which an infra-red light emitting diode (functioning as radiation source) and a photodiode (functioning as detector) are located adjacent to each other and in thermal communication with each other. By locating the source and detector adjacent to each other and in thermal communication, they remain at substantially the same temperature, simplifying the procedure of compensating for temperature variation. Accordingly, some embodiments of the invention aim to provide a compact gas sensor having high sensitivity given the constraint that the source and detector should be either or both adjacent either other and in thermal communication.
Optical absorption gas sensors typically include a mechanism for providing a reference signal, in addition to a measurement signal, to enable more accurate measurement. One mechanism for obtaining a reference signal in a non-dispersive optical absorption sensor is to provide a second detector, with a different filter, to measure light at a wavelength which is not absorbed by any gas which is expected to be present in varying amounts. However, this adds to the complexity of the gas sensor, and therefore its cost. It is known to provide separate measurement and reference radiation sources, but there will typically be different paths between these sources and the detector, or the presence of a reference radiation source will require an optical arrangement which directs a lower proportion of emitted radiation to the detector than would be the case without the presence of a second radiation source. Accordingly, some aspects of the invention aim to provide improved or alternative mechanisms for obtaining a reference signal in an optical absorption gas sensor.